This invention relates to a process for connecting semiconductor chips to circuits on a substrate and an adhesive film used therefor.
As a method for attaching semiconductor chips having electrodes projecting from the main face (such as IC's with bumps) to a wiring substrate, it is known to apply an adhesive between the electrodes of semiconductor chips and the substrate surface where the corresponding circuit terminals are formed, and the IC chips and wiring substrate are pressed against each other so that the electrodes of the IC chips and the wiring substrate will become conductive and be bonded to each other. Regarding the adhesive used for the above purpose, there are a case where an anisotropic electroconductive adhesive prepared by mixing electroconductive particles in an insulating adhesive is used and a case where an insulating adhesive is used.
Anisotropic electroconductive adhesives have a problem in the retention of insulating performance since there may occur short-circuiting due to the presence of electroconductive particles between the adjoining circuits when, for instance, connecting high-density circuits. Also, since the electroconductive particles are generally hard, this type of adhesive involves problems of possible cracking of IC chips in the pressing step at the time of connection and a tendency to cause a rupture in the wiring protective coat.
Regarding a procedure for the application of an insulating adhesive, there is known, for instance, a method in which a low-viscosity curing adhesive such as an ultraviolet-curing type adhesive is applied on the circuits and these circuits are contacted with each other through fine unevenness on the circuit surfaces by pressing at the time of connection to thereby effect desired electrical connection, and then the adhesive is cured to complete the connection and bonding, with the superfluous portion of adhesive being removed out of the circuits (see, for example, Japanese Patent Publication No. 46-43732, and NIKKEI MICRO-DEVICES, June, 1987, issue, page 65, Nikkei McGraw-Hill).
Among these types of adhesives, those made of thermoplastic materials are simple in use for connection but unsatisfactory in heat resistance and connection reliability. Attention is therefore paid to the curing type adhesives.
Curing of adhesive has been generally performed by applying a form of energy such as heat and light (ultraviolet rays, electron rays, etc.). In the case of heat curing, it can be effected by heating and pressing the adhesive between hot plates. In the case of ultraviolet-curing, a transparent plate such as glass plate is used as one of the pressing plates and ultraviolet rays are applied through the transparent plate.
Practical use of said conventional types of adhesives has the problems such as pointed out below.
(1) Because of wide variety in height of projecting electrodes from the main face, a high connection reliability can not be obtained.
The number of projecting electrodes per chip may vary from 10 to as much as 500, and the height of these electrodes is usually in the range of about 1 to 50 .mu.m. It is difficult to form such a large number of electrodes with uniform height, for example, with a scatter of less than 0.5 .mu.m in height. When the height of projecting electrodes is non-uniform, although the electrodes with a large height can easily contact the circuit surface on the substrate, the electrodes with a low height may fail to reach the circuit surface to form a space therebetween, making it unable to obtain an electroconductive connection.
(2) With the method of the type in which a liquid adhesive is applied on the electrodes, it is difficult to control the coated adhesive thickness uniformly, and there may take place such a phenomenon as shortage of adhesive at the connecting portion or generation of air cells, resulting in unsatisfactory connection reliability.
Further, in this type of method, the adhesive used needs to be liquid with low viscosity for the operational reason. Therefore, the adhesive used in this method is usually prepared by using a low-molecular weight material and subjecting it to a curing reaction to turn it into a high-molecular weight substance, or by diluting a high-molecular weight material with a solvent or the like.
In the case of the former method, since a low-molecular weight material is suddenly turned into a high-molecular weight substance, there takes place an excess degree of cure shrinkage, which generates a residual stress at the connecting portion to make its thermal impact resistance unsatisfactory. In the case of latter method, there arises the problem of environmental pollution as the solvent is dried away. Also, the residual solvent is gasified when heat and pressure are applied at the time of connection, causing generation of air cells. It is thus impossible with these methods to obtain satisfactory connection reliability.
(3) When viewed from the adhesive curing system, the ultraviolet-curing method is subject to restriction on the type of substrate employed because the substrate used in this method must be permeable to ultraviolet rays.
It is hardly possible with the heat-curing method to satisfy the two antithetic property requirements, namely keeping quality and quick curing property at the same time. For instance, this method is very poor in performance of such operations as curing by long-time heating under pressure or short-time coating by mixing a curing agent just before use.
In the case of the method using a thermo-plastic material, there is the problem of unsatisfactory heat resistance, and also it is impossible to sufficiently remove the adhesive from the contact area between electrodes and circuits since the drop of viscosity is insufficient under the practical temperature and pressure at the time of connection because of high molecular weight, such as several tens of thousands, of the material, so that a secure electrical connection can not be obtained.
(4) Another problem is the method of regeneration or renovation of faulty parts.
For example, in the case of liquid crystal display module (hereinafter referred to as LCDM), several to several tens of semiconductor chips per LCDM are connected on a transparent electroconductive glass plate (substrate) either directly or through a tape carrier (generally called TAB), and after passing a live test, the assembly is offered as a product module.
Since the live test covers the synthetic tests for semiconductor chips, connected portions, circuitries, etc., it is necessary to conduct the test on the assembly in a state close to that of the finished product.
When an abnormality is detected in the live test, the abnormal part(s), e.g., defective semiconductor chip(s), is(are) replaced with normal one(s), and the connecting operation must be performed again. In this case, it is possible to remove the abnormal part(s) relatively easily when a conventional thermo-plastic adhesive has been used since such adhesive can be lowered in its adhesive force by slight heating. In the case of curing type adhesives, however, since the connected area is firmly bonded and since the adhesive, because of its reticulated (cross-linked) molecular structure, loses little of its little lowered in adhesive force even when heated and the adhesive is hardly soluble in solvents, there are great difficulties effecting the removal of defective part(s).
For removing the defective part(s), therefore, there has been no alternative but to dip the faulty connected portion in a solvent or an acid or alkaline solution for effecting swelling or decomposition of the reticulate structure or to resort to a forcible removing means, such as scraping-off with a knife.
Use of such forcible means, however, may give damage to the normal connections and wiring around the faulty part and would also inevitably leave a part of adhesive unremoved, making it unable to obtain a reconnection with high reliability. Accordingly, renovation has been very difficult and the yield of products would be lowered, leading to an increased production cost.